This figure shows historical Rt estimates. We look back to see when the Rt estimate hits a certain value to indicate the start of a potential COVID wave.For the 2025-2026 season, we saw an increase in the Rt estimate in July(Rt around 1.1) however we NEVER saw a winter wave. Instead, the Rt estimate decreased rapidly starting in August and remained low (below 1.0) for the entire winter. We looked back to see when the Rt increased to 1.0 and then 1.05 to indicate the START of the winter wave for 2022 and 2023. Interestingly, the start of the winter wave happened around the same time in 2022 and 2023 but was later for 2024. For 2024, the Rt hit 1.0 on Oct 26 but NEVER hit 1.05
The Effective Reproductive Number, shown here as “Rt” helps us understand how fast COVID-19 is spreading in our community. For COVID-19, Rt tells us the average number of people who will contract this disease from each infected person.
For example, if Rt equals 1, each existing infection causes one new infection. An Rt equal to 1 means the disease will stay present and stable in our community.
If Rt is less than 1, each existing infection causes less than one new infection. Therefore, if Rt stays below 1, spread of the disease declines and it eventually leaves the community.
Rt can be influenced by variety of factors such as, vaccination, and behaviors like wearing a mask or keeping social distance. This is why Rt can change over time.